As compared with CRT (cathode ray tube), a liquid crystal display device (LCD) is advantageous in that it has a small size and a light mass and consumes a low electric power. The liquid crystal display device comprises a liquid crystal cell and a pair of polarizing plates disposed on the respective side of the liquid crystal cell. The liquid crystal cell comprises rod-shaped liquid crystal molecules, two sheets of substrates for enclosing the rod-shaped liquid crystal molecules therein and an electrode layer for applying a voltage to the rod-shaped liquid crystal molecules. In order to align the rod-shaped liquid crystal molecules thus enclosed in the liquid crystal cell, the two sheets of substrates each have an alignment film provided thereon. In order to decolor the image displayed on the liquid crystal cell, it is normally often practiced to provide an optically-compensatory sheet (retarder plate) interposed between the liquid crystal cell and the polarizing plate. The laminate of the polarizing plate (polarizing film) and the optically-compensatory sheet acts as an elliptical polarizing plate. The optically-compensatory sheet may be rendered capable of expanding the viewing angle of the liquid crystal cell. A stretched birefringence film has heretofore been used as an optically-compensatory sheet.
It has also been proposed that an optically-compensatory sheet having an optically anisotropic layer containing a discotic compound be used instead of stretched birefringence film (see, e.g., JP-A-6-214116, U.S. Pat. Nos. 5,583,679, 5,646,703 and German Patent Application Disclosure No. 3911620). The optically anisotropic layer is formed by aligning a discotic liquid crystal compound, and then fixing the discotic liquid crystal compound thus aligned. A discotic liquid crystal compound normally has a high birefringence. A discotic liquid crystal compound can be aligned in various forms. Accordingly, a discotic liquid crystal compound can be used to produce an optically-compensatory sheet having optical properties that have never been obtained with the prior art stretched birefringence films.
A liquid crystal display device has been proposed comprising a bend-aligned liquid crystal cell having rod-shaped liquid crystal molecules aligned in directions which are opposite from upper to lower side of the liquid crystal cell (symmetrically) (see, e.g., U.S. Pat. Nos. 4,583,825 and 5,410,422). Since the rod-shaped liquid crystal molecules are symmetrically aligned in directions which are opposite from upper to lower side of the liquid crystal cell, the bend-aligned liquid crystal cell has a self optical compensating function. Therefore, this liquid crystal mode is also called OCB (optically compensatory bend) liquid crystal mode. The bend-aligned liquid crystal display device is advantageous in that it exhibits a high response.
As compared with ordinary liquid crystal modes (TN mode, STN mode), the bend alignment mode is characterized by a great viewing angle and a high response. However, the bend alignment mode needs to be further improved as compared with CRT. In order to further improve the bend-aligned liquid crystal display devices, it can be proposed that an optically-compensatory sheet be used as in the ordinary liquid crystal modes. However, the prior art optically-compensatory sheet comprising a stretched birefringence film leaves something to be desired when used in the bend-aligned liquid crystal display device. As previously mentioned, it has been proposed that an optically-compensatory sheet having an optically anisotropic layer containing a discotic liquid crystal compound be used instead of stretched birefringence film. Further, a bend-aligned liquid crystal display device comprising an optically-compensatory sheet containing a discotic liquid crystal compound has been also proposed (see, e.g., JP-A-9-197397 and International Disclosure No. 96/37804). The use of the optically compensatory film containing a discotic liquid crystal compound makes it possible to remarkably improve the viewing angle of the bend-aligned liquid crystal display device.
However, it has been pointed out that the bend-aligned mode liquid crystal display device comprising an optically compensatory sheet containing a discotic liquid crystal compound is subject to leakage of light having a specific wavelength and hence coloration in displayed image (see, e.g., JP-A-11-316378). It is disclosed that this coloration is attributed to the wavelength dependence of transmittance of an ellipsoidal polarizing plate (laminate of polarizing plate and optically compensatory sheet).
It is reported that when the optically anisotropic layer and the polarizing film are disposed such that the angle of the average direction of regular projection of the line normal to the disc surface of the discotic liquid crystal compound toward the optically anisotropic layer with respect to the in-plane transmission axis of the polarizing film is 45°, the maximum optical compensation effect on the bend-aligned mode liquid crystal cell can be exerted. Various methods have been proposed to eliminate tin change and prevent gradation inversion of a bend-aligned liquid crystal device comprising an optically-compensatory sheet containing a discotic liquid crystal compound (see, e.g., Japanese Patent 3056997 and JP-A-2002-40429).